MECHANICAL SEPARATION 281 density. If V is the velocity of discharge and a the area of discharge opening then RA must equal Fa. If V is fixed by the conditions as to head then evidently only one dimension of width of discharge opening will produce satisfactory operation of the thickening tank. If the head of liquid is reduced the discharge opening must be made correspondingly larger. The loss of head room by not using the goose-neck extension only applies where one or a few of the cone, tanks are used. Where a battery of tanks is used the head room required from entry point to lowermost distributing point of thickened material will probably be greater than in the device next to be described. Dorr Thickener.—The tank and the distinctive Dorr mechanism assisting in remvoing the settled solids is shown in Figs. 11 and 12. The essential mechanism is plow blades set at an angle on arms secured to a slowly revolving shaft FIG. 12.—Dorr thickener with double raking device. (from, one revolution in 2 min. to one revolution per hour, the finer and more difficultly settleable the material the slower the rotation). As the mechanism rotates the settled material is pushed towards the discharge point at the center. It is claimed that the rotating mechanism has no harmful effect in stirring up the settling solids and that it assists in densifying the lowermost layers of settled material. Figure 11 shows a tank with single raking and by use of double raking mechanisms as shown in Fig. 12 the capacity of tanks of equal diameter is practically doubled. The tanks are made with the standard mechanism as shown up to 100 ft. and with the use of trays or double mechanisms up to 75 ft. Tanks with special mechanisms have been built up to diameters of 200 ft. If motor-driven, from 2 to 5 hp. should be provided for the standard sizes up to 100-ft. tank diameter. While running the power consumption is very much less than indicated by these motor ratings. Above 100-ft. tanks the power will depend on the layout of the raking mechanism and the makers will have to be consulted.